Which atom in the ground state has a partially filled second electron shell?

hydrogen atom

lithium atom

potassium atom

sodium atom

The answer is lithium.

This is because it has an electron configuration of 1s2, 2s1 (also written as [He]2s1)

Atoms have electrons filled in energy shells.

1. H - hydrogen atom has one electron in the First energy shell. Therefore hydrogen has a partially filled first energy shell

2. Li - Li electron configuration is 2,1

The outermost energy shell is the second energy shell in which there is only one electron

Therefore the second energy shell is partially filled. This is the correct answer

3. K - electron configuration is 2,8,8,1

The outermost energy shell is the fourth energy shell which is partially filled. The second energy shell is completely filled

4. Na - electron configuration is 2,8,1

The outermost energy shell is the third energy shell which is partially filled

Second energy shell is completely filled

From the given options Li is the only element with a partially filled second energy shell

Answer is Li